Positioning two objects

I am having problems positioning object to planes.
If I am creating a building interior in Blender and I have a square planes as floors and walls, and I have a load of objects I want to append to the scene, is there a way I can correctly place or snap objects to my plane without either going through them or leaving them ‘floating’ in air?

Use the magnetic snaps. They are unusual but they work great.
I intend to document them better and if you really want to stick with me I can help you to use them while you will give me an idea of how they are perceived by the user.

You can start with this :

You might try the Drop Onto Ground script in the Object menu also.